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Modem 3600
6-1
Chapter 6
Protocols
CCITT V.42 bis Error Control Protocol
V.42 bis is an industry standard for error control adopted by the
Consultative Committee for International Telephone and Telegraph
(CCITT). The CCITT V.42 bis protocol incorporates two error control
algorithms, LAPM and MNP. LAPM is a CCITT Link Access Protocol
family member related to LAPB and LAPD, currently in use in other
communications applications. MNP is Microcom Networking Protocol,
which has become an industry standard by the large number of its users.
The use of V.42 bis requires both local and remote modems to be V.42 bis
compatible. Error control protocol is transparent to the user and requires
no special hardware or software. Data to be transmitted is put in a buffer
so the modem can retransmit it if an error occurs. The modem also buffers
data received from the remote modem in case an error occurs and the data
is retransmitted. To avoid overfilling the buffer, flow control is used to
control data between the modem and the terminal. V.42 bis protocol
options can be set by AT commands.
Note
Error control protocols are only valid when using asynchronous
DTE options.
Reliable Mode
When an LAPM or MNP link is established, the modem is in reliable
mode. V.42 bis allows negotiation with a remote modem to the highest
level of protocol common to both units. Both LAPM and MNP control
data errors by retransmitting any block of data that was corrupted in
transit. LAPM is assigned highest priority, and, if not supported, then an
MNP connection is attempted.